This is a very interesting topic and actually why criminals prefer cash is because of two reasons mainly a) Because criminals are not educated b) is it easy to spend cash
Now if we take a look at the cyber scams we will notice that the cyber criminals actually ask bitcoins as the extortion money because they know no one operates bitcoins like if they accept Paypal the authorities will easily catch them hence they use bitcoins. So, the direct relation of criminals not accepting crypto is because they are not educated enough while the educated criminal will always prefer bitcoins instead of cash because accepting cash can be problematic for them like police can chase them and catch them but accepting crypto payment eliminates this risk for them.
Exactly. I believe that what's missing here is education. If criminals knew how to use crypto for their own benefit, they wouldn't be using Fiat for all of their nefarious activities. It's just that most criminals are not tech-savvy enough to understand how crypto/Blockchain tech works. I'd prefer them to continue using Fiat for the sake of the entire crypto/Blockchain industry. If criminals start adopting crypto in mass, you'd expect a massive crackdown from mainstream governments. Only then, the growth of the industry will stifle. As much as I want to see crypto succeed, we need to expand it's legitimate use cases for governments to be friendly towards it. But the advent of privacy coins, decentralized exchanges, and even "De-Fi" platforms, might attract wrongdoers into the industry. Governments will try to interfere, but they won't have any luck due to the decentralized nature of crypto/Blockchain tech. After that, they might declare crypto illegal for mainstream use.
Nonetheless, crypto's price volatility, transparency, and steep learning curve will scare off many criminals in the mainstream world. They'll continue to use Fiat because it can be used offline, it's stable, and extremely easy to use for day-to-day transactions. As long as this is the case, there should be nothing to worry about. Let's hope that crypto is used more legitimately for the sake of its growth in the mainstream world. Just my thoughts